Impulse voltage generation devices find applications in inverter drive systems comprising an electric motor, an inverter and a cable. In an inverter drive system, the inverter converts a DC voltage into a pulse voltage by means of a switching operation and supplies the pulse voltage to the motor by way of the cable. The motor is driven to operate by the pulse voltage.
However, in inverter drive systems, reflected waves are produced by impedance mismatching of the inverter, the cable and the motor. As a reflected wave comes to lie on the pulse voltage, high voltage noise can arise between the cable and the motor, particularly at the connecting section where the cable is connected to the motor. Such high voltage noise is referred to as “inverter surge” hereinafter for the purpose of discriminating it from lightning surge.
Tests for evaluating an inverter drive system are known where a simulated inverter surge is generated and applied to the connecting section as load are known. More particularly, there is a known test of repeatedly generating an impulse voltage as simulated inverter surge and alternately providing periods during which an impulse voltage is applied as load and periods during which no impulse voltage is generated. Impulse voltage generation devices that employ discharge gaps have been developed to realize such a test.
The impulse voltage generation device has a high voltage generator, a capacitive element, a first output terminal, a second output terminal, a first electrode and a second electrode.
The high voltage generator is arranged between a first node and a second node. The capacitive element, is arranged in parallel with the high voltage generator between the first node and the second node. Typically, a connecting section of the above-described type is provided between the first output terminal and the second output terminal as load to which an impulse voltage is supplied.
The first electrode and the second electrode are arranged between the first node and the first output terminal. The first electrode and the second electrode are typically spherical metal electrodes (made of tungsten or the like). The first electrode and the second electrode are arranged at positions that are separated from each other.
The high voltage generator generates a high voltage and electric charge is accumulated in the capacitive element due to the high voltages supplied from the high voltage generator. When the voltage between the first electrode and the second electrode gets to the spark discharge triggering voltage level, a spark discharge occurs to generate an impulse voltage between the first output terminal and the second output terminal. The peak value of the impulse voltage is determined by the spark discharge in the atmosphere. It is lower than the high voltage that the high voltage generator supplies.
An impulse voltage generation device that employs a discharge gap generates an impulse voltage by means of spark discharge. Therefore, the parameters including the voltage value of impulse voltage, the rising time, the falling time and the impulse repetition frequency can often fluctuate.
Spark discharge occurs in the atmosphere. Therefore, constant (air) pressure needs to be supplied between the first electrode and the second electrode in order to make the above parameters to be held to respective constant values. However, even if constant air pressure is supplied to between the first and second electrodes, there still exist factors that cannot make the above parameters to be held to constant values.
First, discharge craters appear on the surface of the first electrode and that of the second electrode as a result of spark discharge. Thus, the surfaces of the first and second electrodes need to be cleaned or replaced periodically so as to make the above parameters to be held to constant values.
Second, each time the peak value of impulse voltage is to be adjusted, the distance between the first electrode and the second electrode and hence the discharge gap needs to be adjusted. Since the above parameters change when the discharge gap is changed even slightly, the operation of adjusting the discharge gap is very time consuming.
Therefore, it is difficult for an impulse voltage generation device that employs a discharge gap to repeatedly generate an impulse voltage on a stable basis.
Li Ming et al., “EFFECTS OF REPETITIVE PULSE VOLTAGES ON SURFACE TEMPERATURE INCREASE AT END CORONA FPROTECTION REGION OF HIGH VOLTAGE MOTORS”, 10th Insucon International Conference Birmingham 2006, describes a circuit for generating a high voltage pulse by means of a semiconductor switch. However, the described circuit is not adapted to realize a test of alternately and repeatedly providing periods during which an impulse voltage is generated and periods during which no impulse voltage is generated.